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Old Jul 25, 2010, 07:18 AM
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Question
Inverted 4-stroke problem

I have a World Models Extra 330L. The instructions call for inverting or inverted 45 degree. I have installed a 91 four stroke inverted but am having problems with the engine loading up with fuel causing the engine to backfire when starting. As a test I rotated the engine so that its on its side and I run fine in this orientation. However to fit it nicely inside of the cowl the engine needs to be in either of the orientations mentioned in the instructions. Besides during assemblemby I have cut the cowl to work with the engine inverted.

I know the problem. The carb is below the center line of the fuel tank causing fuel to be syphoned into the carb.

What I am cansidering is using a Perry V20 oscillating pump to eliminate the syphoning. Will this stop the syphoning or will the fuel still be syphoned through the pump?
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Old Jul 25, 2010, 06:43 PM
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Everett Wa.
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"What I am considering is using a Perry V20 oscillating pump to eliminate the syphoning. Will this stop the syphoning or will the fuel still be syphoned through the pump?"

No, the V20 pump will not impede fuel flow in the direction you think you need. Does the engine you have use an auto mixture carb?

Friends don't let friends fly nickel,
Konrad
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Old Jul 25, 2010, 09:20 PM
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The micro oscillating pump does solve your problem I used one for the exact same reason and the engine ran fine.
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Old Jul 25, 2010, 10:13 PM
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Well, lets take a look at the insides of a V20 pump. As you can see there are 3 check valves on the model I have drawn. None of these valves will stop fuel flow from the tank to the carb with the slug motionless. So unless I've mis drawn the pump I don't see how adding the pump will stop siphoning. Can someone please describe how adding the pump will stop siphoning?

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Old Jul 26, 2010, 09:39 AM
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You are siphoning fuel.

Just a thought. Could you put fuel tank in upside down to get tank outlet closer to carb?

Tom
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Old Jul 26, 2010, 09:56 AM
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ps: If you can't get the tank outlet hole level with the carb body, try changing to a slant type tank. May need to prop up or prop down with an extra piece of foam.

Tom
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Old Jul 26, 2010, 10:32 AM
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I just use a pair of hemostats to pinch off the fuel like, until I am ready to fly.

For a while I experimented with a Dubro Quick Fueler, as you could refill the fuel tank and not flood the engine too. Plus when you left the fueler nipple extended or out, it blocked off the fuel going to the carb. But it does cause the fuel lines to be longer and sometimes that can be an issue.
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Old Jul 26, 2010, 12:22 PM
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Quote:
Originally Posted by noveldoc View Post
You are siphoning fuel.

Just a thought. Could you put fuel tank in upside down to get tank outlet closer to carb?

Tom

Changing the position of the outlet of the tank will NOT change the siphon action. It is the level of fuel in the tank in relation to the carb that is important. You have to lower the whole tank to change the static pressure at the carb.
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Old Jul 26, 2010, 01:07 PM
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United States, MI, Marysville
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The check valves in the Sullivan pump will have a static "cracking pressure" This is a function of the exposed valve area and the actual seating pressure . The force X area type of calculation . So long as the differential pressure created by the siphon action is low enough , the check valve can slow or virtually stop the siphon action.
It is not likely to be a "bubble tight" - zero leakage seal but may very well do the trick for you in the "real world".

Hope this helps,

Gary
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Old Jul 26, 2010, 01:08 PM
Heathkit DX-100 son of Bullet
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Old Jul 26, 2010, 01:40 PM
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I have the vp20, it does not regulate so it'll siphon right through. Youd probably want the perry regulating pump. I still think a pump is a good idea though. If its siphoning upright, you'll go massively lean when you fly inverted.
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Old Jul 26, 2010, 02:10 PM
Heathkit DX-100 son of Bullet
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Quote:
Originally Posted by depronair View Post
I have the vp20, it does not regulate so it'll siphon right through. Youd probably want the perry regulating pump. I still think a pump is a good idea though. If its siphoning upright, you'll go massively lean when you fly inverted.
It depends a lot on the installation and the pump itself. If the tank is marginally above tank CL , the siphon pressure action is minimal.

I offered the cracking pressure scenario as a possible (Likely ?) explanation for the earlier "Iluv2fly " reported success with the pump .

A check valve is the electrical equivalent of a diode of course. The cracking pressure is a fairly constant factor roughly equivalent to the forward bias voltage of a diode. It takes a minimal voltage before a diode actually start to conduct . Aside from a floating ,unloaded or inertially actuated check valve, nearly all have a spring action pre-load over a set orfice or valve seat.

The pump also gives you the option of moving the tank to a lower location perhaps further back, nearer the CG. This also minimizes the CG effects of change during the flight.
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Last edited by Gary Cee; Jul 26, 2010 at 02:19 PM.
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